1. Field of the Invention
The present invention relates to an image forming apparatus for forming a color image by driving to cause a plurality of light emitting element arrays to emit light on the basis of color image signals.
2. Description of the Related Art
An example of a system to form color images in the image forming apparatus is a tandem system which provides comparatively high image forming speed. The system is driven specifically by image signals, has components for executing a single-color image forming operation, including a light emitting element array which possesses light emitting elements such as a plurality of LED (light emitting diode) elements, a photosensitive element which receives light from the light emitting element array, a developing device for developing electrostatic latent images formed on the photosensitive element exposed to light from the light emitting element array, a transferring device for transferring a toner image formed by the development onto a specified recording paper, arranged for each color image signal of, for example, yellow (Y), magenta (M), cyan (C) and black (K), respectively, and repeatedly executes operations of formation of electrostatic latent images to at least transfer for each color. An example of the relevant systems is disclosed in Japanese Unexamined Patent Publication JP-A 53-96838(1978). In addition to this, examples using a single photosensitive element are disclosed in Japanese Unexamined Patent Publication JP-A 58-44445 (1983) and JP-A 1-306880(1989).
The image forming apparatus adopting the tandem system comprises comparatively many components, and in order to obtain highly detailed color images, light emitting element arrays themselves or light emitting element array of the same color and each element such as photo-sensitive element must be positioned highly accurately. For example, when one pixel measures approximately 100 .mu.m by 100 .mu.m, each element is positioned so that the positional accuracy of the pixel achieves within .+-.30 .mu.m. However, highly sophisticated techniques are required to carry out this kind of positioning accurately, and this will result in marked rise in the cost of the apparatus, and the large number of components required increases the size of the apparatus.
In addition, the light emitting element array specifically comprises a plurality of LED array chips, and the relevant LED array chips comprise a specified number of light emitting elements, for example, 64 light emitting elements, and LED array chips are chosen from all LED array chips, for example, one by one, and driven in a time-sharing manner to form color images. For example, when four light emitting element arrays are used to obtain four-color images, since the respective light emitting element array are formed on separate substrates, the circuit for choosing the LED array chip is formed on each substrate. That is a cause of upsized apparatus. In addition, it is relatively complicated to control time-sharing drive by the four chosen circuits.